this is actually something Rust specifically tried to avoid with GC, that is, not having the ecosystem split into GC-world and non-GC-world, and arguably something D failed at
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Replying to @whitequark @mcclure111
one thing that makes this worse is that type systems bolted on top of dynlangs tend to be unsound, e.g. mypy is
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Replying to @whitequark
Yes. And generics systems on statically typed languages tend to be designed around the assumption there's a JIT
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Replying to @mcclure111
you can go a long way with inline caches, see ObjC
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Replying to @whitequark @mcclure111
Hm, the approaches I know of to add inline caches or method caches to ObjC all ended up getting rolled back because they didn’t make things faster enough to justify the code size and memory cost. ObjC is weird though because when you care about perf you just…don’t use objects
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I think the biggest benefit isn’t so much ICs but speculative devirtualization leading to opportunistic inlining, which is really tough without a heat profile of the code (PGO or JIT)
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Yeah that’s pretty much exactly the problem with ObjC. PGO is of course hard to do well, and people generally manually optimize out the dispatch in known hot paths anyway
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Like, modern CPU’s BTB will mostly do a good job with virtual call overhead. (objc_msgSend is weird but I’m guessing it’s keyed off {pc,lr} on Apple’s chips or something to deal with that.) But inlining, that obviously opens up arbitrarily many optimizations.
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Which is probably why Microsoft invested so early in PGO. Just the ability to inline IUnknown methods (QueryInterface(), AddRef(), Release()) in COM-heavy code seems huge…
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How well does Microsoft's profiler-guided optimization work in practice? I know they've done a lot of it but I don't think I know anyone who's ever used it.
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We use it in Firefox. It’s worth a 10% perf boost or so as I recall
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wait didn't u… write ur own programming language… specifically to write Firefox in
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This decision predates the use of Rust in Firefox, and there’s a whole lot of C++ still there :) Browser C++ code tends to use a lot more virtual dispatch than necessary (especially Gecko with its XPCOM, though this has improved). It’d probably help less in Rust.
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